Rotary indexing table driven by an induction motor

ABSTRACT

The present invention is a rotary indexing table driven by an induction motor. The rotary indexing table includes a rotatable work supporting platform, an AC induction motor including a motor shaft coupled to the rotatable work supporting platform; and a controller operatively coupled to the AC induction motor. The AC induction motor is equipped with a high resolution positional feedback device. The high-resolution positional feedback device may be an encoder or a resolver. The controller is configured to drive the AC induction motor in a direct drive manner. The high-resolution positional feedback device is operatively coupled to the controller, and the controller is configured to filter a signal provided by the high-resolution positional feedback device. The signal provided by the high-resolution positional feedback device may be a square wave or a sine wave. The present invention is also directed to a method precisely driving and positioning a rotary indexing table.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to rotary tables. More particularly,the present invention relates to a rotary indexing table utilizing an ACinduction motor with a high-resolution positional feedback device.

2. Description of Related Art

Rotary tables, such as rotary indexing tables, are well-known for theaccurate positioning of work pieces at work stations for automatedoperations. Rotary indexing tables typically have a table and an indexerassembly that rotates the table through a predetermined angle forpositioning work pieces for sequential automated operations.

Rotary indexing tables have been successfully employed in the field ofautomated assembly for work stations including pick and place devices,feeder bowls, visual inspections, label applicators, robot arms,adhesive applicators, laser machining and other automated assemblyprocesses. Rotary indexing tables are further well-known in the fieldsof machining for the accurate positioning of work pieces to receivedrilling, boring, tapping, CNC machining, facing, grinding, and othertypes of machining processes. Other uses for rotary indexing tablesinclude the accurate positioning of work pieces for coating,sterilizing, cleaning, testing and calibrating.

As described in U.S. Pat. No. 5,950,503, rotary indexing tables havealso been used in the decorating field for screen printing, hotstamping, pad printing, ink jet printing, impact marking, laser marking,spray painting and other decorative processes. For example, rotaryindexing tables are currently employed for multi-color screen printingonto work pieces such as CD's, credit cards, key fobs, etc. Typically, arotary indexing table supports multiple, equidistantly positionedfixtures. The fixtures receive and support the work pieces during theprinting operations. At a first work station, a work piece isautomatically positioned onto the fixture. The table then rotatesthrough a precise angle or distance to position the work piece under afirst screen printing apparatus. After the printing is completed, thetable rotates through the same angle again to position the work piecefor receiving a second overlaying screen print image. The indexingprocess continues until the work piece has received all the requiredlayers of screen printing and is removed from the fixture at a finalwork station.

With the need for very precise machining and close tolerances inmanufacture, rotary indexing tables have had to be much more precise andprovide more through-put in order for the industry to remaincompetitive. Rotary indexing tables, for example, may be required tomove through a complex set of rotary profiles such as continuousrotation, indexing with a dwell time, oscillation, variable speed orreverse direction. It would be advantageous to have an assembly capableof all these motions while maintaining precision. In addition, with theadvent of robotics these assemblies are required to place a work pieceat various work angles relative to the work station to provide accessfrom automated operational equipment.

Typically, prior art rotary indexing tables, also known as turntables,are centrally driven and work is performed at the periphery of thetable. Alternately, when tables are driven on their outside diameter,the drive mechanism tends to be outside the periphery of the table andthus impedes use of the assembly in various angles and in operationswhere space is at a premium.

Most prior art rotary indexing tables are driven by cams or genevamechanisms through a speed reducer and electric motor. Rotary indexingtables of this type suffer from various drawbacks including a fixednumber of index positions, the inability to provide continuous rotationand the inability to be programmed.

Another prior art method of driving a rotary indexing table utilizes aring gear and pinion arrangement powered with a speed reducer andelectric motor. This method, however, also suffers from a variety ofdrawbacks. For instance, the use of a ring gear and pinion arrangementhas a lower precision due to backlash. Also, such arrangements are verycostly.

A third prior art method of driving a rotary indexing table is throughthe use of a servomotor configured to drive a cam or pinion gear. Theuse of a servomotor is costly and also requires a large number ofmechanical components. Furthermore, servomotors usually require aload-to-motor inertia mismatch that is very low, such as 10 to 1. If theload-to-motor inertia mismatch exceeds this requirement, the result isinstability and poor performance.

A final prior art method for driving a rotary indexing table is the useof a low speed direct drive permanent magnet motor. Such direct drivepermanent magnet motors, however, are very expensive. Furthermore, thebearing loads of such motors limit the use of overhung loads on thetooling ring.

Accordingly, a need exists for a rotary indexing table with a drivemechanism that is of low cost while still providing high precision. Afurther need exists for a rotary indexing table with a simplified designincluding few moving parts so as to reduce backlash. A final need exitsfor a drive mechanism for a rotary indexing table that provides accuratepositioning and smooth motion in the presence of very high inertialloads.

SUMMARY OF THE INVENTION

The present invention is a rotary indexing table driven by an inductionmotor. The rotary indexing table includes a rotatable work supportingplatform, an AC induction motor including a motor shaft coupled to therotatable work supporting platform; and a controller operatively coupledto the AC induction motor. The AC induction motor is equipped with ahigh resolution positional feedback device. The high-resolutionpositional feedback device may be an encoder or a resolver. Thecontroller is configured to drive the AC induction motor in a directdrive manner. The high-resolution positional feedback device isoperatively coupled to the controller, and the controller is configuredto filter a signal provided by the high-resolution positional feedbackdevice. The signal provided by the high-resolution positional feedbackdevice may be a square wave or a sine wave.

The present invention is further directed to a rotary indexing table forsupporting workpieces and moving the workpieces through a plurality ofpositions. The rotary indexing table comprises a rotatable,substantially planar, circular work supporting platform; a directlydriven AC induction motor including a motor shaft coupled to therotatable work supporting platform and a high resolution positionalfeedback device; and a controller operatively coupled to the ACinduction motor and high-resolution positional feedback device. Thecontroller is configured to filter a signal provided by thehigh-resolution positional feedback device.

The present invention is further directed to a method of preciselydriving and positioning a rotary indexing table. The method includes thesteps of providing a rotary indexing table including a rotatable worksupporting platform; an AC induction motor including a motor shaftcoupled to the rotatable work supporting platform and a high-resolutionpositional feedback device; and a controller operatively coupled to theAC induction motor. Next, the rotatable work supporting platform isdriven using the AC induction motor and the high-resolution positionalfeedback device. Then a feedback signal is provided by thehigh-resolution positional feedback device, and the controller filtersthe feedback signal to produce a filtered signal. Finally, the ACinduction motor is controlled to position the rotatable work supportingplatform based on the filtered signal.

These and other features and characteristics of the present invention,as well as the methods of operation and functions of the relatedelements of structures and the combination of parts and economies ofmanufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention. As usedin the specification and the claims, the singular form of “a”, “an”, and“the” include plural referents unless the context clearly dictatesotherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a rotary indexing table driven by an ACinduction motor with a high-resolution positional feedback device inaccordance with the present invention;

FIG. 2 is a side plan view of the rotary indexing table driven by an ACinduction motor with a high-resolution positional feedback device inaccordance with the present invention;

FIG. 3 is a cross-sectional view taken along line 3′-3′ in FIG. 2;

FIG. 4 a is a top plan view of a second embodiment of the rotaryindexing table in accordance with the present invention;

FIG. 4 b is a side plan view of the second embodiment of the rotaryindexing table in accordance with the present invention;

FIG. 5 a is a top plan view of a third embodiment of the rotary indexingtable in accordance with the present invention;

FIG. 5 b is a side plan view of the third embodiment of the rotaryindexing table in accordance with the present invention;

FIG. 6 a is a top plan view of a fourth embodiment of the rotaryindexing table in accordance with the present invention; and

FIG. 6 b is a side plan view of the fourth embodiment of the rotaryindexing table in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “lateral”,“longitudinal” and derivatives thereof shall relate to the invention asit is oriented in the drawing figures. However, it is to be understoodthat the invention may assume various alternative variations, exceptwhere expressly specified to the contrary. It is also to be understoodthat the specific devices illustrated in the attached drawings, anddescribed in the following specification, are simply exemplaryembodiments of the invention. Hence, specific dimensions and otherphysical characteristics related to the embodiments disclosed herein arenot to be considered as limiting.

With reference to FIGS. 1-3, a rotary indexing table 1 includes arotatable work supporting platform 3. Rotatable work supporting platform3 is used to support work pieces, tooling, fixtures and the like forpositioning as is known in the art. Rotatable work supporting platform 3is rotationally mounted to a drive hub 5, which is in turn coupled to adrive end 7 of a motor shaft of an AC induction motor 9. Rotary indexingtable 1 is also configured to include a bearing 11 secured by a flangemount bearing plate 13. Bearing 11 allows for smooth rotation of worksupporting platform 3.

The use of AC induction motor 9 is advantageous because constructioncosts are relatively low compared with other types of motors, and ACinduction motors are very reliable. In general, an AC induction motorincludes a stator and a rotor. In operation, a rotating magnetic fieldis generated in the stator, which induces a magnetic field in the rotor.The two fields interact and cause the rotor to turn. To obtain maximuminteraction between the fields, a very small air gap is provided betweenthe rotor and stator. The speed of the rotor depends upon the torquerequirements of the load. The bigger the load, the stronger the turningforce needed to rotate the rotor. The turning force can increase only ifa rotor-induced electromagnetic field increases. This electromagneticfield can increase only if the magnetic field cuts through the rotor ata faster rate. To increase the relative speed between the field androtor, the rotor must slow down. Therefore, for heavier loads theinduction motor turns slower than for lighter loads. Furthermore, ACinduction motor 9 may be directly driven. This is advantageous becausethe use of a directly driven induction motor eliminates the need forgearing or belting thereby simplifying the design and reducing backlash.Furthermore, the elimination of gearing and belting also allows themotor to provide stable performance even in the presence of largeinertial loads.

Rotary indexing table 1 further includes a high-resolution feedbackdevice 15 coupled to the opposite end 16 of the motor shaft using amounting plate 17 and an adapter shaft 19. High-resolution positionalfeedback device 15 may be an encoder, a resolver or the like. Theresolution of high-resolution positional feedback device 15 is desirablybetween 1,000,000 and 5,000,000 counts per revolution allowing thedevice to more accurately represent the actual speed of AC inductionmotor 9. A controller 21 is operatively coupled to AC induction motor 9and high-resolution positional feedback device 15. Controller 21 may becoupled to AC induction motor 9 and high-resolution positional feedbackdevice 15 by an electrical connection, a wireless connection or anyother suitable connection means. Controller 21 is configured to includefeedback signal filtering capabilities.

The combination of high-resolution positional feedback device 15 withthe process of filtering the high-resolution positional feedback signalwith controller 21 allows rotary indexing table 1 to run smoothly with avery high degree of accuracy. First, the use of high-resolutionpositional feedback device 15 is critical to the operation of rotaryindexing table 1. It provides the required accuracy and fine resolutionfeedback required to properly move and position rotary indexing table 1.A high-resolution feedback signal reduces speed feedback ripple byallowing controller 21 to more accurately reflect the actual speed of ACinduction motor 9. The high-resolution feedback signal may be asquare-wave, a sine wave or the like. Next, it is important thatcontroller 21 is configured to include filtering capabilities. Byfiltering the high-resolution feedback signal, an even smoother feedbacksignal is created, which minimizes or eliminates erratic motion,improves stability and allows very high load-to-motor inertiamismatches.

In many applications, the inertial load of rotary indexing table 1 maybe in excess of 100 times the motor inertia. In order to achieveaccurate positioning and smooth motion given such high inertial loads,controller 21 must operate with very high gain. In other words,controller 21 must be capable of providing large corrections inposition, speed and torque for small differences between a commanded andactual position, speed and torque. If these large corrections are notprovided, stability problems arise.

While the previous embodiment has been described in terms of a directlydriven rotary table, the present invention may also be driven throughthe use of gearing, belting or any other suitable means. With referenceto FIGS. 4 a and 4 b, a second embodiment of a rotary indexing table 40with an open center driven by an AC induction motor 41 and agear-to-gear drive 42. Rotary indexing table 40 further includes arotatable work supporting platform 43 used to support work pieces,tooling, fixtures and the like for positioning as is known in the art.Gear-to-gear drive 42 includes pinion gear 44 and a main gear 45. Piniongear 44 is coupled to a drive end of a motor shaft of AC induction motor41. A drive force is provided by AC induction motor 41 to pinion gear 44thereby causing main gear 45 to rotate. The rotation of main gear 45provides rotation to rotatable work support platform 43.

Rotary indexing table 40 further includes a high-resolution positionalfeedback device 46 coupled to the opposite end of the motor shaft of ACinduction motor 41 using a mounting plate 47. A controller 48 isoperatively coupled to AC induction motor 41 and high-resolutionpositional feedback device 46. Controller 48 may be coupled to ACinduction motor 41 and high-resolution positional feedback device 46 byan electrical connection, a wireless connection or any other suitableconnection means. As discussed above, controller 48 is configured toinclude feedback signal filtering capabilities.

The combination of high-resolution positional feedback device 46 withthe process of filtering the high-resolution positional feedback signalwith controller 48 allows rotary indexing table 40 to run smoothly witha very high degree of accuracy as discussed in detail above reference toFIGS. 1-3.

With reference to FIGS. 5 a and 5 b, a third embodiment of a rotaryindexing table 50 with an open center is driven by an AC induction motor51 and a belt drive 52. Rotary indexing table 50 further includes arotatable work supporting platform 53 used to support work pieces,tooling, fixtures and the like for positioning as is known in the art.Belt drive 52 includes a toothed pulley 54, a belt 55 and a main pulley56. Toothed pulley 54 is coupled to a drive end of a motor shaft of ACinduction motor 51. A drive force is provided by AC induction motor 51to toothed pulley 54 thereby causing force to be exerted on belt 55causing main pulley 56 to rotate. The rotation of main pulley 56provides rotation to rotatable work support platform 53.

Rotary indexing table 50 further includes a high-resolution positionalfeedback device 57 coupled to the opposite end of the motor shaft of ACinduction motor 51 using a mounting plate 58. A controller 59 isoperatively coupled to AC induction motor 51 and high-resolutionpositional feedback device 57. Controller 59 may be coupled to ACinduction motor 51 and high-resolution positional feedback device 57 byan electrical connection, a wireless connection or any other suitableconnection means. As discussed above, controller 59 is configured toinclude feedback signal filtering capabilities.

The combination of high-resolution positional feedback device 57 withthe process of filtering the high-resolution positional feedback signalwith controller 59 allows rotary indexing table 50 to run smoothly witha very high degree of accuracy as discussed in detail above withreference to FIGS. 1-3.

With reference to FIGS. 6 a and 6 b, a final embodiment of a rotaryindexing table 60 is driven by an AC induction motor 61 and a gearhead62. Rotary indexing table 60 further includes a rotatable worksupporting platform 63 used to support work pieces, tooling, fixturesand the like for positioning as is known in the art. Gearhead 62 iscoupled at a first end to rotatable work supporting platform 63 and at asecond end to a drive end of a motor shaft of AC induction motor 61. Adrive force is provided by AC induction motor 61 to gearhead 62 therebycausing force to be exerted on rotatable work support platform 63causing it to rotate.

Rotary indexing table 60 further includes a high-resolution positionalfeedback device 64 coupled to the opposite end of the motor shaft of ACinduction motor 61 using a mounting plate 65. A controller 66 isoperatively coupled to AC induction motor 61 and high-resolutionpositional feedback device 64. Controller 66 may be coupled to ACinduction motor 61 and high-resolution positional feedback device 64 byan electrical connection, a wireless connection or any other suitableconnection means. As discussed above, controller 66 is configured toinclude feedback signal filtering capabilities.

The combination of high-resolution positional feedback device 64 withthe process of filtering the high-resolution positional feedback signalwith controller 66 allows rotary indexing table 60 to run smoothly witha very high degree of accuracy as discussed in detail above withreference to FIGS. 1-3.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims. For example, it is to be understood thatthe present invention contemplates that, to the extent possible, one ormore features of any embodiment can be combined with one or morefeatures of any other embodiment.

1. A rotary indexing table comprising: a rotatable work supportingplatform; an AC induction motor including a motor shaft coupled to therotatable work supporting platform; and a controller operatively coupledto the AC induction motor, wherein the AC induction motor is equippedwith a high resolution positional feedback device.
 2. The rotaryindexing table of claim 1, wherein the high-resolution positionalfeedback device is an encoder.
 3. The rotary indexing table of claim 1,wherein the high-resolution positional feedback device is a resolver. 4.The rotary indexing table of claim 1, wherein the controller isconfigured to drive the AC induction motor in a direct drive manner. 5.The rotary indexing table of claim 1, wherein the high-resolutionpositional feedback device is operatively coupled to the controller. 6.The rotary indexing table of claim 5, wherein the controller isconfigured to filter a signal provided by the high-resolution positionalfeedback device.
 7. The rotary indexing table of claim 6, wherein thesignal provided by the high-resolution positional feedback device is oneof a square wave and a sine wave.
 8. A rotary indexing table forsupporting workpieces and moving the workpieces through a plurality ofpositions comprising: a rotatable, substantially planar, circular worksupporting platform; a directly driven AC induction motor including amotor shaft coupled to the rotatable work supporting platform and a highresolution positional feedback device; and a controller operativelycoupled to the AC induction motor and high-resolution positionalfeedback device, wherein wherein the controller is configured to filtera signal provided by the high-resolution positional feedback device. 9.The rotary indexing table of claim 8, wherein the high-resolutionpositional feedback device is an encoder.
 10. The rotary indexing tableof claim 8, wherein the high-resolution positional feedback device is aresolver.
 11. The rotary indexing table of claim 8, wherein the signalprovided by the high-resolution positional feedback device is one of asquare wave and a sine wave.
 12. A method of precisely driving andpositioning a rotary indexing table comprising: providing a rotaryindexing table including: a rotatable work supporting platform; an ACinduction motor including a motor shaft coupled to the rotatable worksupporting platform and a high-resolution positional feedback device;and a controller operatively coupled to the AC induction motor; drivingthe rotatable work supporting platform using the AC induction motor andthe high-resolution positional feedback device; receiving a feedbacksignal from the high-resolution positional feedback device; filteringthe feedback signal to produce a filtered signal; and controlling the ACinduction motor to position the rotatable work supporting platform basedon the filtered signal.
 13. The method of claim 12, wherein thehigh-resolution positional feedback device is an encoder.
 14. The methodof claim 12, wherein the high-resolution positional feedback device is aresolver.
 16. The method of claim 12, wherein the signal provided by thehigh-resolution positional feedback device is one of a square wave and asine wave.
 17. The method of claim 12, wherein the filtering stepinvolves smoothing the feedback signal.